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<html lang="en">
  <head>
    <meta charset="UTF-8" />
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    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <button type="button">关灯</button>
    <script>
      window.onload = function () {
        document.querySelector("button").onclick = function () {
          if (this.innerText === "关灯") {
            document.body.style.backgroundColor = "black";
            this.innerText = "开灯";
          } else if (this.innerText === "开灯") {
            document.body.style.backgroundColor = "#fff";
            this.innerText = "关灯";
          }
        };
      };
    </script>
  </body>
</html>
